May Sinclair's father was Scottish and her mother was Irish.
May Sinclair was born Mary Amelia Saint Clair Sinclair on August 23, 1863, in Rock Ferry, Cheshire, United Kingdom to the family of a Liverpool shipowner William Sinclair and Amelia Hind. She was the sixth child and only daughter.
Amelia Hind was puritanical; William Sinclair was a heavy drinker and womanizer. He had inherited a shipping business but in 1870, when Sinclair was seven, he went bankrupt. The family broke up, never to reunite, and from then on she lived in a succession of different parts of England with her mother, suddenly forced into genteel poverty. She was always secretive about her early years and biographers have found it difficult to unravel the details. Several have inferred its outline from her later novel Mary Olivier, however, which appears to be strongly autobiographical. It implies that she was involved in a prolonged love-hate relationship with her mother and that, thirsty for the formal education that had been granted to her brothers but denied her, retreated into books and learning.
All her brothers were heavy drinkers and all died prematurely, as did her estranged father, leading her to expect that her heredity probably condemned her too to a premature grave.